FLOOD03929 - Laserfiche WebLink

Home Browse Search

Simulated river channel changes are associated with the gradual re- duction of the spatial variation of energy gradient along the channel sub- ject to the physical constraint of rigid banks, That the adjustment in river-channel configuration is closely related to the change in power ex- penditure can be illustrated by the sequential changes of cross-sectional profile at Section 51 as shown in Fig. 9. Because it is an initial sand ridge (see Fig. 7), the energy gradient at this section is initially much greater than those of its adjacent sections. This pronounced spatial variation in energy gradient is reduced through gully erosion at this section and deposition at the adjacent sections. The gully which is small in width and has a low channel-bed elevation provides the least possible flow resistance and therefore lowest energy gradient at this section; it also reduces the back-water effect on the upstream section where the energy gradient is therefore increased. At subsequent time intervals, the energy gradient at Section 51 becomes less than its adjacent sections. Cross- sectional changes at this section then include widening in channel width and aggradation in the gully. These changes are accompanied by in- creases in boundary resistance and energy gradient at this section, fa- voring the establishment of equal energy gradient along the reach. This pattern of river channel changes, characterized by the formation of nar- row ~hannel width during channel-bed degradation and widening dur- ing aggradation, is evident in nature and has been reported in the lit- erature (4,5,11,13,15). Changes in Sediment and Hydraulic Parameters.-That flood- and sediment-routing in erodible channels is closely related to river-channel changes may be illustrated by the time and spatial variations of the ve- locity and sediment load shown in Fig. 7 at the peaks of the first and the second flood. The pronounced spatial variations in velocity and sed- iment load at the first peak flood are associated with the uneven river channel configuration dotted with borrow pits within which velocities and sediment loads are substantially lower. Changes in the river channel are such that they provide the mechanism to establish the dynamic equi- librium of sediment transport, that is, equal sediment load along the river reach. As shown, the spatial variation of sediment load is gradually reduced during the second flood. The same general trend may also be stated for the spatial variations in velocity and energy gradient. The slightly lower velocity at the bridge crossing is due to the additional flow resis- tance of bridge piers. Variations of sediment size due to hydrauliC sorting as simulated are not pronounced for this river reach. Certain trends can still be recog- nized, including coarsening of the material during scour and reduction in size during deposition. Channel widening through bank erosion brings finer bank materials into the channel and hence contributes to a reduc- tion in sediment size. Water-Surface Profiles.-The water-surface profile at the peak flood obtained using the FLUVIAL-11 model is compared with that obtained using the fixed-bed model HEC-2 in Fig. 7. The HEC-2 profile which is based upon the initial river-channel configuration indicates critical flow at Sections 46, 51, and 60, and subcritical flow at remaining sections, while the FLUVIAL-11 model predicts subcritical flow for the entire reach. The HEC-2 water-surface profile is highly uneven; the higher water-sur- 169 18